I have tried couplers and I found that the best way is to just epoxy the prop on the shaft. I use a file to cut groves in the shaft to allow the epoxy to better grip it others just sand it which should work just fine. I have extra motors and props ready to go in case I should need one. The couplers add more length and you may get more wobble in the prop which does not seem to happen as much when I epoxy them. PS I never did find a good coupler to adapt the motor shaft to the 4mm prop.